The present invention pertains to optically active 1-amino-2-(4-hydroxyphenyl) ethylphosphonic acid and its derivatives thereof. The compounds of the present invention are useful as intermediates for the synthesis of phosphorus-containing oligopeptides having hypotensive activity such as those compounds disclosed in the aforementioned, commonly owned, related applications. 1-amino-2-(4-hydroxyphenyl) ethylphosphonic acid in racemic form is a known compound and is described in Chemical Abstracts 62, 6510e (1965). In addition, Austrian Pat. No. 342,192 discloses compounds represented by the formula: ##STR1## wherein R.sub.a is hydrogen, lower alkyl, lower cycloalkyl, lower cycloalkyl-lower alkyl, aryl or aryl-lower alkyl where these residual groups may be substituted by one or more of amino, hydroxy, thio, methylthio, carboxyl or guanidino; R.sub.b and R.sub.c are residual groups of .alpha.-amino acid; R.sub.d is hydroxyl or methyl; and n is 0,1,2 or 3. The carbons indicated by one asterisk have L-configuration, and the carbon indicated by two asterisks has R configuration; and according to the patent, the compounds are useful for increasing the antibacterial activity of cephalosporin or penicillin antibiotics, etc.